The head-to-tail condensation of isopentenyl pyrophosphate with dimethylallyl pyrophosphate and geranyl pyrophosphate and the head-to-head condensation of farnesyl pyrophosphate to yield squalene are the major building reactons in the sterol biosynthetic pathway. This proposal is concerned with the topology of the substrates in the enzyme substrate complex for farnesyl pyrophosphate synthetase during the head-to-tail condensation and with the development of novel inhibitors of squalene synthetase. In both instances, molecules will be synthesized which covalently link species not joined but bound at the same time during the enzymatic reactions. We will sythesize a novel class of bisubstate analogues for farnesyl pyrophosphate synthetase which links the allylic and homoallylic substrates by a covalent bridge. These modifications will restrict the topologies of the partners to a narrow ranges of reactive conformations. It will be possible to deduce the topology of the bound substrates by determining how the analogues function as alternate substrates and inhibitors. Several important clues about the catalytic process can be deduced from the structural information, including the nature of the base that assists with proton removal in the final step. Model studies are also planned to see if regio and stereoselectivity can be conferred by interactions between the pyrophosphate moieties in the analogues and magnesium. Cyclopropylcarbinyl cation and inorganic pyrophosphate ion pairs are thought to be reactive intermediates during the rearrangement of presqualene pyrophosphate to squalene, the second step in the head-to-head condensation catalyzed by squalene synthetase. By linking both species in a single molecule we should be able to synthesize a powerful inhibitor of the enzyme which may be a selective agent for regulating the biosynthesis of cholesterol without perturbing the remaining branches of the terpene pathway.